In a vehicle such as a one-box car, a sliding door is widely configured as a door used to open and close an entrance provided on a flank of a vehicle body in a vehicle width direction. Such a sliding door is generally configured to slide in a vehicle front and rear direction in a location rearward of a pillar with respect to the vehicle so that opening and closing a doorway serve as the entrance, and furthermore, the pillar is made up of an outer panel and an inner panel. When the sliding door is closed, a door holding mechanism installed in a front portion of the pillar engages with an engaging member installed in a rear end portion of the sliding door, and thereby the sliding door is held. For example, a structure according to Japanese Patent Laid-Open No. 2010-083167 is configured as follows: a center rail configured to guide a sliding door by extending in a vehicle front and rear direction, is mounted on a lateral surface of an outer panel (side panel) in a vehicle width direction, a door holding mechanism (opening prevention bracket) is installed in a front end portion of the center rail located in front portion of a pillar, and when the sliding door is closed, the door holding mechanism holds the sliding door by engaging with an engaging member (filler lid stopper) of a door opening and closing mechanism which is installed in a rear end portion of the sliding door.
However, when forces are applied from outside of the vehicle, surroundings of the door holding mechanism may be deformed by the applied forces, and as a result, the engaging member of the closed sliding door may be disengaged from the door holding mechanism on a vehicle-body side. Consequently, there is demand to increase rigidity around the door holding mechanism. One example of structures which increase rigidity around conventional door holding mechanisms includes a structure according to Japanese Patent Laid-Open No. 2003-175859. In the structure according to Japanese Patent Laid-Open No. 2003-175859, an outer panel and an inner panel extending substantially in an upper and lower direction are formed into a pillar having a hollow section, a striker is mounted on a front face of a front portion of the pillar, and a pillar reinforcement having substantially L-shaped form in a cross section is arranged inside the hollow pillar, wherein a front portion of the pillar reinforcement extends in a vehicle width direction, a rear portion of the pillar reinforcement extends in a straight line from a lateral end, in the vehicle width direction, of the front portion obliquely to an outer side in the vehicle width direction, a front portion of the pillar reinforcement is mounted on a rear face of the front portion of the pillar on which the striker is mounted, and a rear end of the pillar reinforcement is joined to an inside surface of a lateral portion, in the vehicle width direction, of the outer panel, the outer panel extending toward the rear portion of the vehicle from the front face of the pillar.